The demand for multi-lateral drilling is increasing as more horizontal wells are being used on development wells to improve reservoir drainage and productivity. Drilling multi-lateral wells provides the advantage that the productivity or recovery of hydrocarbons from a formation can be improved by drilling a number of branch wells, all of which feed a mother well. Drilling multi-lateral wells can also reduce the platform cost by reducing the number of slots required to exploit a given hydrocarbon formation.
In order to drill a multi-lateral well, it is known to mill a window in the casing of the mother well, and deflect a drilling tool through the window to drill the lateral well by using a deflection tool which is commonly known as a "whipstock".
By using this conventional technique, lateral wells can be completed "barefoot" (that is no loner pipe is used to case off the lateral well). Barefoot completions are only satisfactory where the formations drilled are suitable for such a completion (such as a limestone formation).
However, where sands are encountered in the pay zone of the formation, which is very common, the well must be cased and perforated to allow production of the hydrocarbons to take place without the lateral well "sanding off" or collapsing. In addition, many wells are geo-pressured which requires that the well is cased off and a pressure seal is effected at the junction of the lateral well liner and the mother well casing. Conventionally, cement is used to effect the pressure seal at the junction. However, cement seals may be relatively unreliable or create a relatively low pressure resistant seal.
In order to insert casing into the lateral well, several techniques are known. For instance, it is known to have a casing with a section of side wall which incorporates an outlet tube and which is pivotably coupled to the casing by a hinge mechanism. The pivotable section is moveable from a first position in which the outer surface of the casing is cylindrical and the inner bore of the casing is substantially narrowed by the outlet tube; and a second position in which the inner bore of the casing is cylindrical and the outlet tube protrudes from the casing.
However, this type of arrangement has the disadvantage that the diameter of the lateral well is limited with respect to the diameter of the mother well due to the pivotable outlet tube arrangement, as it must be contained within the main casing during installation.
A further disadvantage in the drilling of lateral wells from a mother well is that the maximum bend for conventional drilling assemblies is about 15 degrees per hundred feet. Thus, the initial (maximum) deviation angle of a lateral well from the mother well is limited to approximately 4.degree. to 5.degree.. This angle limitation therefore requires a large window in the side of the casing through which the drilling assembly will exit the casing, and this window may typically be 12 feet long.